An example of an optical disk is shown in FIG. 7. This disk is provided with a recording layer in which lands L and grooves G are formed in an alternating fashion along the tracking direction Tg (radial direction of the optical disk). A push-pull method, for example, is available as a method for performing tracking control in an optical disk device that uses such an optical disk. The interference of light reflected the optical disk is indispensable for understanding the present invention which utilizes this method, and is therefore described below.
As shown in the same figure, when a beam of light is projected to the grooves G of the optical disk, the reflected light includes a bundle of zero-order light rays R0 and two bundles of first-order light rays R1 interfering with each other to generate interference light Ia. The zero-order light R0 consists of non-diffracted light that is reflected so as to follow the irradiating path of the light beam toward the grooves G. In contrast, the two bundles of first-order light rays R1 consist of positive and negative first-order diffracted light rays that are generated due to the fact that the lands L and the grooves G are arranged in side-by-side relationship in the tracking direction. When the beam of light is irradiated at a position offset from the center of the groove G in the tracking direction, the two bundles of first-order light rays R1 become asymmetric and the reflected light develops differential intensity in the tracking direction. The reflected light is detected using an optical detector 9 having two light-receiving sections 90e, 90f arranged in the tracking direction Tg, as shown in FIG. 8 for example. Electrical signals Se, Sf each having a level corresponding to the amount of received light (light intensity) by the light-receiving sections 90e, 90f are outputted from the optical detector 9, and a tracking error signal (push-pull signal) is created by taking the difference of the two electrical signals. This tracking error signal represents the direction and amount of the tracking error.
The track pitch need be reduced with a resulting increase of the data recording density in order to increase the data recording capacity of the optical disk. However, as the track pitch is reduced, the interval between the two bundles of first-order light rays R1 widens, so that the proportion of interference light Ia contained in the reflected light reduces. Eventually leads, a so-called diffraction limit is reached in which no interference light Ia is contained in the reflected light. The limit track pitch Pt that causes the diffraction limit is theoretically defined as Pt=λ/NA/2 (where λ is the wavelength of the light beam and NA is the numerical aperture of the objective lens). It becomes difficult to detect a tracking error by the above-mentioned method when the track pitch of the optical disk is smaller than the limit track pitch described above.
A conventional counter-measure is disclosed in JP-A 2000-331383. According to the conventional counter-measure, grooves Gd, Gt adjacent to each other in the tracking direction Tg are designed to differ from each other with respect to one or both of the depth and width, as shown in FIG. 9. The grooves Gd, Gt are formed to extend along two co-extensive spirals as shown in FIG. 10A, or to extend along a single spiral in which they are alternately connected to each other as shown in FIG. 10B.
According to this prior art technique, the apparent spatial frequency of the optical disk can be made ½. Consequently, a tracking error can be detected even when the track pitch is made narrower than the limit track pitch established by the equation Pt=λ/NA/2 above.
However, the prior art technique described above has the following drawbacks. In the former structure shown in FIG. 10A, the two grooves Gd, Gt extend along two parallel spirals, so when data are continuously written in the tracks of the optical disk, for example, processing for writing to the groove Gd and processing for writing to the groove Gt must be alternately performed. The control of continuous writing while changing the writing-target tracks in this manner is not easy, and its implementation is difficult. In order to form the grooves Gd, Gt by a depicting method that uses an electron beam in the manufacture of the optical disk, two electron beams must be used, so the optical disk is also difficult to manufacture.
In contrast, in the latter structure shown in FIG. 10B, the grooves Gd, Gt are formed along a single spiral, so the grooves can be formed using a single electron beam, and the groove forming operation is facilitated. Write processing is also facilitated because there is no need to change the writing-target tracks when data are continuously written in the tracks. However, the latter structure still has the following drawbacks.
Firstly, it is not easy to perform control whereby the optical head is held facing the same track during idle time in which no data are written or read. The reason for this is that because the tracks extend along a spiral, the optical head cannot be held facing the same orbital track solely by performing tracking control on the basis of the tracking error signal obtained by the above-described method, and control for track jumps, referred to as “track jumping”, must be performed.
Secondly, both grooves Gd, Gt are equally interrupted at the borderline indicated by the line L1 shown in FIG. 10B. Therefore, a sudden change occurs whereby the polarity of the tracking error signal obtained at that time is reversed when the beam of light continues to irradiate the section near the changeover point. Consequently, when the optical head attempts to find the number of traversed tracks by counting the number of zero points in the tracking error signal during a seek operation for moving the optical head to a position opposite the target track, counting errors can easily occur due to erroneous counting of the sudden change in the tracking error signal described above.
Thirdly, the sensitivity with which the tracking error is detected is by no means high, and it is difficult to perform highly accurate tracking control on the basis of the tracking error signal. This third drawback is also encountered in the former structure shown in FIG. 10A.